Preparation of monocrystalline rutile



PREPARATION OF MON OCRYSTALLINE RUTILE Malcolm D. Beals, Fanwood, N. J.,and James Fill filler, Lancaster, Ohio, assignorsto National LeadCompany, New York, N. Y., a corporation of New Jersey No Drawing.Application August 12, 1953, Serial No. 373,884

4 Claims. (Cl. 106-42) The present invention relates to rutile singlecrystal boules modified with respect to color and certain other physicalproperties, and to methods for the production of such boules.

Rutile is one of the three known crystal modifications of titaniumdioxide. When substantially pure, a massive single crystal of rutile hasgem-like properties with a very light straw color and reflectance,refraction and brilliance greater than that of a diamond. By means ofthe present invention rutile single crystals may be produced which havea nearly water-white color. In addition, the present invention permitsfinishing the boule by reoxidation in considerably less time than isrequired for a pure rutile single crystal boule.

The principal object of the present invention is the preparation ofrutile single crystal boules having nearly water-white color. Anotherobject of the invention is a method for producing a finished, oxidizedrutile single crystal boule in considerably less time than is requiredfor the preparation of a finished oxidized pure rutile single crystalboule. These and other objects of the present invention will becomeapparent from the following descriptron.

In its broadest aspect the present invention contemplates a rutilesingle crystal boule containing a small amount of an oxidic compound ofgallium in solid solution and charabterized by having a nearlywater-white color. This water-white single crystal may be preparedaccording to the method of the present invention byprogressively fusinga mixture of finely divided TiOz containing a small amount of an oxidiccompound of gallium in a flame carrying an excess of an oxidizing gasand thereafter oxidizing boule so formed.

In this description of the present invention the word boule is used inits currently accepted meaning to denote a characteristic shape or formof an artificially prepared massive single crystal having a rounded end,or meniscus, a more or less rod-like body portion and a tapering end,giving an overall appearance in profile of an almond, or carrot.

The general method to which the presentinvention relates is thatdescribed in another of assignees applications, that is in co-aendingapplication Serial No. 286,853, filed May 9, 1952, and now abandoned,for the preparation of pure rutile single crystal boules.

According to the preferred method of the present invention, there isemployed as starting material a substantially pure TiOz which is veryfine, fairly uniform and possesses an open structure with units capableof being rapidly melted. A TiOz having an ultimate unit particle size ofapproximately 0.1 micron has proved especially satisfactory. In general,material having an average particle size above about 5 microns should beavoided because such particles do not satisfactoriiy fuse under theconditions of the invention. A preferred method for preparing a T iOzstarting material especially adapted for the production of rutile singlecrystal boules is described in U. 8. Patent No. 2,521,392, issuedSeptember 5, 1950,

wherein the double salt ammonium-titanium-sulfate is calcined until allof the ammonia and sulfate has been removed by volatilization.

Substantially any oxidic compound of gallium'can be added to the feedmaterial, for example, the sesqui-oxide or any oxidic compounds whichform the oxide at the temperatures employed in the preparation of therutile boules.

According to the present invention the small amount of gallium compoundmaybe mixed with the TiOz starting material in any convenient manner,for instance, by dry mixing. A more uniform mixture and, therefore, apreferred mixture may be prepared by adding gallium oxide as a powder,slurry, or solution to the double salt, ammonium-titanium-sulfate priorto calcination.

The amount of gallium compound to be added to the TiOz starting materialshould not exceed about 1.0%, calculated as gallium oxide based on theweight of the TiOz. The lower limit which is effective is about 0.05%.When the amount of gallium oxide is increased substantially above about1.0% resulting boules tend to fracture and this is to be avoided. Withamounts of gallium oxide of about 1.0% and below a complete solidsolution of the gallium oxide in the TiOz is formed from which a highquality unfractured boule is produced.

In progressively fusing the starting mixture of "H02 and gallium oxidethere should be employed a flame which carries an excess of an oxidizinggas, preferably oxygen. Preferably the fusion is carried out accordingto the method described in application Serial No. 286,853 supra, inwhich the particles of the starting mixture are entrained in a stream ofoxygen in a tube constituting the center of a burner consisting of threeconcentric tubes. In this burner hydrogen is introduced to the flamethrough the intermediate tube and additional oxygen in excess over theamount required to combine with the hydrogen is introduced through theouter tube. Preferably, the ratio of oxygen to hydrogen by volume shouldbe about 1:1 and should not be less than about 8.5 parts of oxygen to9.5 parts hydrogen by volume.

The excess of oxygen in the flame is necessary because at elevatedtemperatures, titanium dioxide gives up oxygen and is converted tolower, or sub-oxides of titanium, e. g. TiaOs. This loss of oxygenproceeds with considerable rapidity under conditions produced by anoxy-hydrogen flame wherein the TiO2 is fused and the loss is acceleratedwhen the flame is neutral or reducing. Under such conditions theformation of lower, or sub-oxides of titanium, is so great that mixturesof small crystals of various oxides of titanium are produced rather thana single crystal boule.

Even when operating with an excess of oxygen the boule as produced has agenerally deep blue-black color indica- I tive of some deficiency inoxygen. Therefore, in order to produce the nearly water-white boule ofthe present invention, the blue-black boule has to be re-oxidized.

The reoxidation of the boule is preferably carried out by heating theblue-black boule in an oxidizing atmos phere, for instance, air oroxygen. The temperature of the treatment should be within the range fromabout 650 C. to about 1500 C. It has been found that temperatures to anyextent below 650 C. oxygen will not be appreciably incorporated into therutile single crystal while temperatures above 1500 C. are expensive anddifficult to maintain. The heating should be continued until the bouleobtains the maximum degree of lightness of color, this time will usuallybe about one-quarter of that required to fully oxidize a pure rutileboule.

As produced, the gallium oxide containing rutile boule has a more orless frosted outer surface. When split, the interior surfaces of thepieces are vitreous and shiny and nearly water-white. They may be cutand polished to form beautiful gems, more brilliant than diamonds andhaving a white color compared to the characteristic straw or lightyellow color of pure rutile.

The foregoing description has been given for purposes of illustration,but the invention is not to be limited thereto and various modificationsand variations Within the scope of the appended claims maybe employed.

We claim:

1. A single crystal of synthetic rutile consisting essentially oftitania and from about 0.05% to about 1.0% of an oxidic compound ofgallium.

2. A single crystal of synthetic rutile consisting essentially oftitania and from about 0.05% to about 1.0% gallium oxide.

3. A method for synthesizing a substantially waterwhite single crystalof rutile consisting of fusing a powder containing titania and fromabout 0.05% to about 1.0% of an oxidic compound of gallium,crystallizing the fused material and heating the crystallized materialin an oxidiz ing atmosphere of oxidizing gas until the water-white coloris obtained.

4. The method according to claim 3 in which the crystallized material isheated in an atmosphere of oxygen at a 10 temperature between about 650C. and 1500 C.

No references cited.

1. A SINGLE CRYSTAL FO SYNTHETIC RUTILE CONSISTING ESSENTIALLY OFTITANIA AND FROM ABOUT 0.05% TO ABOUT 1.0% OF AN OXIDIC COMPOUND OFGALLIUM.